The early prior art of automotive test equipment is characterized by the use of separate test instruments, such as ammeters, tachometers and oscilloscopes. The operator uses the oscilloscope to view automotive signals in real time The more modern art features computer-based testers that integrate the individual test instrument functions. These testers have a video monitor for display of test procedures and results. Also, the testers often provide a digital scope capability in which signal waveforms are displayed in the traditional oscilloscope format.
However, display inaccuracies occur in modern testers due to the limited resolution of the video monitor (typical horizontal resolution being 640 picture elements (pixels)). Specifically, this occurs when digitizing and displaying a high frequency portion of a sensed engine signal (e.g., the portions of the primary and secondary ignition voltage signals occurring upon and after spark and dwell) in conjunction lower frequency portions of the signal. Often, the digitization apparatus includes an analog to digital converter that samples the signals at a higher rate than the monitor resolution. Thus, to obtain an accurate representation of the entire signal (e.g., one cycle) on the display screen, it becomes necessary to decide which samples to keep and which samples to ignore.